diff -r c8023561505b src/jdk/nashorn/internal/runtime/PropertyListenerManager.java
--- a/src/jdk/nashorn/internal/runtime/PropertyListenerManager.java	Thu May 02 15:01:16 2013 -0300
+++ b/src/jdk/nashorn/internal/runtime/PropertyListenerManager.java	Fri May 03 10:49:07 2013 +0200
@@ -61,7 +61,7 @@
      *
      * @param listener The property listener that is added.
      */
-    public final void addPropertyListener(final PropertyListener listener) {
+    public synchronized final void addPropertyListener(final PropertyListener listener) {
         if (listeners == null) {
             listeners = new WeakHashMap<>();
         }
@@ -77,7 +77,7 @@
      *
      * @param listener The property listener that is removed.
      */
-    public final void removePropertyListener(final PropertyListener listener) {
+    public synchronized final void removePropertyListener(final PropertyListener listener) {
         if (listeners != null) {
             if (Context.DEBUG) {
                 listenersRemoved++;